So why should Sony not take the best out of two worlds, keep the SLT for the great EVF and constant lifeview, but when you press the shutter it would flip up and give way for a free sensor?

Well I don't quite understand this point. The main issue people has with EVF as opposed to an OVF is that the image is not exactly what gets through the lens by getting all the light that comes though the it.

And one the strongest points for EVF is that you see what the sensor sees all the time. And include any changes that might have been done by the camera software.

So you are suggesting a setup that will take away one of the advantages of EVF - as the sensor will get a bit more light when shooting - while not gaining the advantage the OVF has.